ᐅ Low water flow in the shower

Dear forum,

Since we moved into our house back in 2016, I have often been annoyed by the low flow rate in our showers.
For your information, the bathroom is located on the first floor, and a Grohe Euphoria 180 is installed.
The water pressure while showering is quite low, which is not too problematic when using the "rainfall" setting, but when using the hand shower, it feels like very little water comes out of the line. Our builder says this is due to the shower’s water-saving features.
I have measured the flow rate at various points in the house and wonder if this is really the cause, or if the pipes were simply sized too small.

Here are the values I recorded (100% cold water):
Kitchen: 6.5 L/min (1.7 gallons/min)
Bathroom bathtub faucet: 10.5 L/min (2.8 gallons/min)
Bathroom shower hand shower: 8.5 L/min (2.2 gallons/min)
Bathroom sink (2 faucets on one connection): 8.5 L/min (2.2 gallons/min)
Garage 3/4": 21 L/min (5.5 gallons/min)

According to the pressure gauge after the backflow preventer, the pressure is 3.5 bar (51 psi). Increasing it is not possible.
It should be noted that the household water passes through an ion exchanger, which, according to the datasheet, should provide 8 m³/h (35 gallons/min). However, only 1/2" braided steel hoses are connected before and after it. Since there is a mixing valve installed, I can shut off the ion exchanger, and the flow rates then are as follows:

Bathroom bathtub faucet: 14 L/min (3.7 gallons/min)
Bathroom shower hand shower: 9 L/min (2.4 gallons/min)
Bathroom sink: 10 L/min (2.6 gallons/min)

This suggests that the ion exchanger at least partially restricts the cold water flow. Would it make sense to replace the 1/2" braided steel hoses with 3/4" or even 1" ones in that section?
For comparison, a “standard” faucet in the kitchen typically has 3/8" pipes — are these flow values plausible for that size? Is it possible to use larger-diameter pipes here as well?

Is there anything else that I could test? What options do I have to increase the pressure in the shower? Could it also be possible that the pressure feels lower because the water is simply “softer”? The water hardness is set at 4° dH (4 °dH).

Any tips are greatly appreciated.
Best regards

Before increasing all the pipes, I would consider a rain shower head for cost reasons alone, as it can provide a better sense of volume while using less water. There are definitely differences in this.

Try checking if it makes a difference by unscrewing the handheld showerhead from the hose and measuring directly from the hose. Many showerheads and handheld showers also have an eco version (including from Grohe). If this does not make a difference, you can already rule out the shower fixtures.

Test that with the main shower as well, and also at shower temperature, not using cold water only.

If you unscrew the showerhead, there might be a reducer piece between the showerhead and the hose.

Addendum: the aerators are usually water-saving aerators. Check the flow at the washbasin without the aerator.